Elastomer gas pressure seals have long been used in a variety of applications including, for example, confining breathing air inside space vehicles. Various silicone compounds are predominantly utilized for these seals because of their temperature performance and reusability. Unfortunately, however, silicone seals have a high relative leak rate due to the permeability of these silicone compounds. The overall leak rate rises even further when the sealing surface is degraded from such things as, for example, on-orbit atomic oxygen impingement, ultraviolet radiation exposure, micrometeoroids and orbital debris impacts, and foreign object debris and lunar dust contamination.
Particularly in aerospace applications, where the leak rate performance of the gas pressure seals dictates the quantity and weight of resources required to replenish breathing air, it is critically important to understand and quantify the leak rate and corresponding mass loss of an escaping gas or gases. For each seal design considered for use in space, for example, lengthy and costly developmental programs are commonly undertaken to quantify the leak rate within the seal's anticipated operational envelope to determine both the conditions of maximum leak rate and the leak rates at common operational conditions. Since multiple tests may be conducted to achieve statistical significance or to reduce experimental uncertainty, large quantities of leak rate tests are often required to achieve meaningful results.
Many methods are known in the art for measuring gas (or other fluid) leak rates of various magnitudes. For gas seals used in space habitat applications, two methods are most typically used, depending upon the size of the leak and the gas to be measured. The first is a pressure decay method with a mass point leak rate analysis. In this method, the test apparatus consists of a volume of gas or other fluid that is hermetically sealed except for the article being tested (referred to herein as a “nominally closed system”). The apparatus is pressurized with an ideal gas until the pressure is slightly above a desired differential pressure (see, FIGS. 1A-B), which is then allowed to leak from a high pressure side through the test article to a low pressure side. The gas pressure and temperature are recorded over time as the gas leaks through the testing article, causing the pressure upstream of the test article to decrease. The reduction in the high-side pressure continues until the desired pressure differential is achieved. The leakage is allowed to continue for some underdetermined internal pressure loss beyond the target differential pressure. The mass of gas within the system is calculated using a thermodynamic equation of state such as the ideal gas law. The mass of the gas is then calculated at an arbitrary number of time-steps yielding a mass-time data set (ti,mi). A best-fit line to the data is computed using a linear least-squares regression centered about the differential pressure of interest. The first-order coefficient of the best-fit line represents the leak rate of the test article.
The pressure decay method with mass point leak rate analysis may be applied to a wide range of leak rates, and a reasonable measurement uncertainty can be achieved. The method utilizes low cost equipment, mainly temperature and pressure measurement devices, and any ideal gas, fluid/ideal gas mixture, or compressible liquid of interest may be characterized. This is important because different gases leak at different rates through similar size and configuration leakage paths.
Prior art mass point leak rate methods generally utilize one of two downstream pressure configurations. In one configuration, atmospheric laboratory pressure conditions are downstream of the test article. (See e.g., FIG. 1A). As such the downstream barometric pressure is variable or quasi-static depending upon the duration of the test and the weather conditions. The second typical configuration is similar to the first but utilizes a constant downstream pressure, typically vacuum pressure provided by a vacuum pump. (See e.g., FIG. 1B) To achieve this downstream pressure, another seal is installed to create a region that is capable of confining the vacuum pressure. The difference between these two applications of the traditional mass point leak rate method is minor and the theory and calculations are identical for both.
There are, however, a variety of significant problems with known pressure decay methods with a mass point leak rate analysis. The first major problem with these prior art methods is that to accurately quantify the leak rate of the nominally closed system, the leak rate must be known (or closely estimated) a priori because the internal pressure and internal test section volume influence the pressure decay rate. In general, the system is designed around a desired leak rate at a specified pressure differential, as the leak rate increases with increasing pressure differential. If the magnitude of the leak rate is too great with respect to the design of the rest of the system, the test's differential pressure crosses the target differential pressure too quickly and the amount of usable data surrounding the crossover point is limited. See FIG. 2. Conversely, if the magnitude of the leak rate is too small with respect to the design of the rest of the system, the test differential pressure approaches the target differential pressure very slowly, (see, FIG. 3) and a long time is needed to reach the desired pressure differential. During this time period, test temperature and/or pressure outside the closed system may vary excessively degrading the data.
To avoid these problems, it is necessary when using the traditional method to know the leak rate of the nominally closed system before the leak rate test is configured and started. The consequences of having to know the test result value before starting the test are twofold. To begin with, the test must be run twice. The first time, a leak rate measurement identifies the leak rate, but is not likely to be of the highest quality. Adjustments are then made to the measurement system and the test is rerun. In addition, any unexpected results generated during the first test are likely attributed to the lower quality of the measurement and may be dismissed. If there are any anomalies that occur during the first test and are not confirmed by the second test, then a third test may be required.
Secondly, with these methods, mass loss and measurement uncertainty cannot be calculated in real time because the test operator must determine which subset of the collected data should be used for mass loss computations. Data is recorded throughout the duration of the test. The test operator determines when the data collection should stop and then which subset of the collected data should be used for mass loss computations. The retained data subset is centered at the pressure differential of interest. The quantity of data that is contained within the retained subset is determined by the operator. It is this step that prohibits the existing method of computing the mass loss and measurement uncertainty in real-time as data collection must be complete before the mass loss and measurement uncertainty may be calculated.
Third, in these prior art methods, the full data set is parsed such that a portion of the data at the beginning of the data and a portion of the data at the end of the data set are discarded. This operation is undesirable as it may involve manipulation and computations on large data sets consuming long time durations, both computationally and for the operator. The discarding of data collected represents inefficiency in this method. While uncommon, at this point the operator may realize that the test was started or ended at inappropriate test conditions resulting in the test being rerun. Fourth, with these prior art methods, each test quantifies only one leak rate. The method cannot quantify leaks that vary with time. Any variation with time is averaged into the test result. Last, while these methods do take temperature into account, the methods remains temperature sensitive. The temperature used in the calculations must accurately represent the gas temperature, which may be challenging to accomplish. Moreover, the duration of the leak test may vary depending upon the combination of test article leak rate and the size of the internal volume.
A second commonly used leak rate quantification method is the helium (or other tracer gas, such as argon or neon) leak detector method. Using this method, the apparatus is pressurized with helium on the interior of the test article. The low-pressure side of the test article is vacuum pressure, typically less than 10 mtorr, and is connected to a mass spectrometer leak detector. The gases that are transported past the test article are electrically charged in the detector. The detector's mass spectrometer associates the abundance of tracer gas ions with a volumetric flow rate. The tracer gas method's main advantage is that the leak rate is computed in real-time, but this advantage comes with several significant limitations.
To begin with, the types of gases that may be used as tracer gases are limited, (commonly helium, argon, or neon), with the type of gas affecting the leak rate value. Particularly in aerospace applications involving confining breathing air inside space vehicles, it is desirable to determine the leak rate of air (rather than a tracer or component gas) through the test article. The leak rate for air (or another target gas) may be significantly different than that of the tracer gas. The leak rate from a helium leak detector test is reported in volumetric flow rate of helium which has to be converted to mass flow rate of air for application to space habitat seals; the conversion process is not constant, nor trivial, as it is dependent upon the geometry of the leak paths within the test article. In addition, since the mass spectrometer used to count the number of tracer atoms/molecules that have escaped the closed volume can only function under vacuum conditions, the downstream pressure must be a vacuum pressure. Also, while this test technique has been found to provide excellent results for very small volumetric flow rates, it becomes impractical for large flow rates since the pressure at the mass spectrometer can exceed operational limits when too much tracer gas flows into the leak detector. This impracticality includes situations when the size of the seal becomes too great, as the leak rate increases linearly with seal length for seals of identical geometry and material (e.g., O-rings). Moreover, the tracer gas leak detector method requires expensive equipment and properly trained personnel. Further, while the test duration is not necessarily lengthy, the detector calibration process must be completed during every test trial, extending the length of the test.
Moreover, the measurement uncertainty for these tracer gas methods is difficult to compute, uncontrollable, and may be orders of magnitude higher than that generated using the mass point leak rate techniques described above. The difficulty in computing the uncertainty arises from the fact that it is dependent upon the test setup. It is uncontrollable because the uncertainty is dependent upon the test data and is computed after the conclusion of the test; the test cannot be run such that a defined value of measurement uncertainty is achieved. Due to the relatively poor repeatability and accuracy of the mass spectrometer, the measurement uncertainty can be very large (greater than 100%). As will be apparent, reducing margins of error by utilizing test methods with quantifiable and low measurement uncertainty is important for obtaining statistically significant and useful results and these are features that these tracer gas methods do not provide.
For the reasons set forth above, among others, known leak rate quantification methods have significant drawbacks, particularly when used to develop space seal performance data. What is needed in the art is a leak rate quantification apparatus and method having all of the desired attributes described above, namely the capability to: quantify small and large leak rates, assign a reasonable level of uncertainty, utilize air as the test gas, and maintain short test durations with leak rates and associated uncertainty calculated in real-time, without the drawbacks of current methods.